1. Field of the Invention
This invention relates to electronic keyboard operated musical instruments and in particular is concerned with the starting phase assigned to generated tones.
2. Description of the Prior Art
Electronic musical instruments all too frequently are designed with tonal characteristics which differ with related tonal characteristics associated with either a single acoustical musical instrument or with a small group of acoustical musical instruments. It is virtually impossible for a group of such musical instruments to initiate their tones all at the same relative phase. On the other hand, the mechanical-like tone generators of many electronic keyboard musical instruments provide a ready means for initiating all tones with a precise common relative phase.
The majority of current commercially available electronic musical instruments use a tone generation system that has been given the generic name of "top octave synthesizer." In these instruments the 12 top octave musical note frequencies are generated by means of counters which count down from a single master clock at prescribed integer ratios. The remainder of the keyboard frequencies are obtained by a sequence of counters which divide by 2,4,8,16 . . . . It is a characteristic of a top octave synthesizer that the octaves are locked in phase in the sense that the fundamental of a note is in phase with the second harmonic of a note an octave below the first note. Systems having this phase characteristic of octavely related notes is said to have "locked octaves."
A positive attribute of musical instruments having phase locked octaves is that when two notes are played at an octave spacing, the resultant tone increases in loudness as a simple sum because there is no phase cancelation of harmonics. A strong negative attribute of phase locked octaves is that the result of two octavely related notes is a steady tone which completely lacks the beating effect characteristic of playing two acoustic musical instruments. It is this beating effect that provides a desired "warmth" to the ensemble tone of a combination of musical instruments. If two acoustical instruments happen to initiate their individual tones either exactly in phase or exactly out of phase, the initial phase situation is short-lived because of the lack of precise tuning of the instruments and because of the center frequency instability that is characteristic of most acoustical musical instruments.
Attempts have been made to decouple, or unlock, the octaves in electronic musical instruments by employing a frequency generating system that deliberately detunes each keyboard note by a set of predetermined frequency errors. While such systems provide a satisfactory steady state ensemble effect they do not address the problem of a random initial phase associated with the actuation of a keyboard switch. If the frequency error is made large enough to rapidly change the relative phases of the generated tones then one may find that the out-of-tuneness can be more objectionable than the tonal effect produced by locked octaves.